Not just boys, my 5yo girl is fascinated by them.

I'm pretty sure there's already a popular hypothesis that relates to evolutionary biology, but I don't know the name of it.

Essentially, as hunter-gatherers every group of humans needed one or more experts on the local megafauna, so virtually all of us have some degree of fascination with big animals baked in, but some have more than others. I've also heard the same hypothesis used to express young boys' obsessions with trucks and trains.

My young lad went thru a brief phase where we went thru picture books of common vehicles and for page after page, he pointed at every wheel of every vehicle and had me name it. "Wheel." "Wheel." "Wheel." ...

Probably a developmental phase somehow connected to quadrupeds. Large object of interest. Four supporting thingies.

Quite easy explanation: dinosaurs are super awesome.

In the ancient times, those who didn't pay enough attentions to dinosaurs in their childhood did not grow old enough to reproduce.

Boys yearn for power - Giant monsters, fast cars, off-highway trucks, guns, dirt bikes and skateboards and anything that jumps, fighter jets with missiles, martial arts, whistling into a telephone to get free calls...

(Girls surely do too, but I cannot attest to that)